Ubiquitination is a post translational modification initially identified as a crucial component of proteasomal degradation in the ubiquitin proteasome system (UPS). Chains of Ubiquitin (Ub(s)), an 8.5 kDa highly conserved protein, are covalently attached to substrates to be degraded in the proteasome. (Finley D. “Recognition and processing of ubiquitin-protein conjugates by the proteasome.” Annual review of biochemistry 78:477-513, (2009)) The molecular mechanisms by which the UPS acts are also varied, with different chain linkages of ubiquitination controlling protein turnover, enzymatic activity, subcellular localization, and protein-protein interactions of substrate proteins. (Komander D., et. al. “The emerging complexity of protein ubiquitination,” Biochem. Soc. Trans. 37 (Pt 5):937-53 (2009))
Ubiquitin-specific protease 7 (USP7) is a Ubiquitin Specific Protease (USP) family deubiquitinase (DUB) that was originally identified as an enzyme that interacted with virally-encoded proteins of the Herpes simplex virus and later the Epstein-Barr virus. (Everett R. D., Meredith M., Orr A., Cross A, Kathoria M., Parkinson J. “A novel ubiquitin-specific protease is dynamically associated with the PML nuclear domain and binds to a herpes virus regulatory protein,” EMBO J. 16(7):1519-30 (1997); Holowaty M. N., Zeghouf M., Wu H., et al. “Protein profiling with Epstein-Barr nuclear antigen-1 reveals an interaction with the herpesvirus-associated ubiquitin-specific protease HAUSP/USP7,” J. Biol. Chem. 278(32):29987-94 (2003)) Ubiquitin Specific Proteases (USPs) specifically cleave the isopeptide bond at the carboxy terminus of ubiquitin. In contrast to other DUB classes, which are thought to generally regulate ubiquitin homeostasis or to be involved in pre-processing of linear ubiquitin chains, USPs remove ubiquitin from specific targets. Given this substrate specificity combined with the numerous roles ubiquitination has in the cell, USPs are important regulators of a multitude of pathways, ranging from preventing the proteolysis of ubquitinated substrates, to controlling their nuclear localization.
USP7 deubiquitinates a variety of cellular targets involved in different processes related to cancer and metastasis, neurodegenerative diseases, immunological disorders, osteoporosis, arthritis inflammatory disorders, cardiovascular diseases, ischemic diseases, viral infections and diseases, and bacterial infections and diseases.
For example, USP7 has been shown to stabilize DNMT1, a DNA methyltransferase that maintain epigenetic silencing, to maintain higher steady state-levels of Claspin, a protein involved in ataxia telangiectasia and Rad3-related (ATR) phosphorylation of Chkl, and to regulate Tip60 protein levels, a histone acetyltransferase and transcriptional coregulator involved in adipogenesis. (Zhanwen du, Song J., Wang Y., et al. “DNMT1 stability is regulated by proteins coordinating deubiquitination and acetylation-driven ubiquitination,” Science Signaling 3 (146) (2010); Faustrup H., Bekker-Jensen S., Bartek J., Lukas J., Mail N., Mailand N. “USP7 counteracts SCFbetaTrCP—but not APCCdh1-mediated proteolysis of Claspin,” The Journal of cell biology 184(1):13-9 (2009); Gao Y., Koppen A., Rakhsh M., et al. “Early adipogenesis is regulated through USP7-mediated deubiquitination of the histone acetyltransferase TIP60,” Nature Communications 4:2656 (2013)
In addition to regulating the protein stability of poly-ubiquitinated targets, USP7 also acts to control the subcellular localization of proteins. Mono-ubiquitination of PTEN has been shown to effect its cytoplasmic/nuclear partitioning, where nuclear localization of PTEN is important for its tumor suppression activity. (Trotman L. C., Wang X., Alimonti A., et al. “Ubiquitination regulates PTEN nuclear import and tumor suppression,” Cell 128(1):141-56 (2007); Song M. S., Salmena L., Carracedo A., et al. “The deubiquitinylation and localization of PTEN are regulated by a HAUSP-PML network,” Nature 455(7214):813-7 (2008)) USP7 has also been shown to bind and deubiquitinate FOXO4, a member of the FOXO subfamily of transcription factors involved in a variety of cell processes including metabolism, cell cycle regulation apoptosis, and response to oxidative stress, decreasing its nuclear localization and transcriptional activity. (van der Horst A., van der Horst O., de Vries-Smits A. M. M., et al. “FOXO4 transcriptional activity is regulated by monoubiquitination and USP7/HAUSP,” Nat. Cell Biol. 8(10):1064-73 (2006))
Cellular targets of USP7 also include the tumor suppressor p53 and its major E3 ligase, MDM2, stabilizing p53 via the degradation of MDM2. (Li M., Chen D., Shiloh A., et al. “Deubiquitination of p53 by HAUSP is an important pathway for p53 stabilization,” Nature 416(6881):648-53 (2002); Li M., Brooks C. L., Kon N., Gu W. “A dynamic role of HAUSP in the p53-Mdm2 pathway,” Mol. Cell. 13(6):879-86 (2004)) Structural studies have also shown that the EBNA1 protein encoded by the Epstein-Barr virus interacts at the same binding surface as USP7 on p53, preventing USP7 endogenous cellular activity while recruiting USP7 to viral promoters in order to activate latent viral gene expression. (Saridakis V., et al. “Structure of the p53 binding domain of HAUSP/USP7 bound to Epstein-Barr nuclear antigen 1 implications for EBV-mediated immortalization,” Mol. Cell. 18(1):25-36 (2005); Sarkari F., Sanchez-Alcaraz T., Wang S., Holowaty M. N., Sheng Y., Frappier L. “EBNA1-mediated recruitment of a histone H2B deubiquitylating complex to the Epstein-Barr virus latent origin of DNA replication,” PLoS pathogens 5 (10) (2009); Sheng Y., et al. “Molecular recognition of p53 and MDM2 by USP7/HAUSP,” Nat. Struct. Mol. Biol. 13(3):285-91 (2006)) Similarly, the gene product of TSPYL5, a gene frequently amplified in breast cancer and associated with poor clinical outcome, alters the ubiquitination status of p53 via its interaction with USP7. (Epping M. T., et al. “TSPYL5 suppresses p53 levels and function by physical interaction with USP7,” Nat. Cell Biol. 13(1):102-8 (2011))
Inhibition of USP7 with small molecule inhibitors therefore has the potential to be a treatment for cancers and other disorders. For this reason, there remains a considerable need for novel and potent small molecule inhibitors of USP7.